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  1. #1
    Join Date
    Nov 2009
    Location
    Melbourne, FL
    Posts
    44

    How do I know how big a system I need?

    I recently received quotes from three different companies. Two of the companies said I needed a 5 ton system, while the other guy swears up and down I only need a 4 ton system.

    Of the two who say I need a 5 ton system, only one did a heat load calculation. He showed me his worksheet.

    The guy who says I need a 4 ton system didn't mention anything about a heat load calculation while he was here, although he did take awhile to put quotes together (like maybe a half an hour? Not nearly as long as the guy I saw take measurements) and he did go up into my attic. When I told him about the other guy who did a load calculation, he told me he did one as well (though it was never mentioned in any conversation). He said he did it on his computer and was skeptical when I mentioned that the other guy did it with a pencil/paper worksheet. I asked him to send me the heat load calculation and he has yet to do so.

    For what it's worth, my house is roughly 2450 sf. It faces east. I have three large sliders that make up the majority of the backside of my house, but I also have a large back porch with a roof, so I don't get much sun or heat in the back of the house. The guy who showed me his calculations said he wrote up my sliders as having window tinting. They don't have it, but he said he wrote it up like that to take into account the shading I receive from my porch roof.

    My current system is 18 years old and the info on it has worn off, so no one knows what I currently have.

    What would you be inclined to think?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,743
    Might be better if you did your own load calc.
    Use This its HVAC Calc. It has a 49 dollar fee. But is well worth it.

    Then you can ask your contractors why theirs is different(which ever one is).
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  3. #3
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,842
    Did either one tell you exactly what the heat gain calcs were? Since AC sizing goes from 4 to 5 ton the 5 ton quote may have resulted from a load calc that was slightly over the 4 ton limit and he bumped up one size to make sure it's covered (a two stage unit may be a better choice). Load calcs are only as good as the program used and the information input. Trash in, trash out. The more sophisicated the load calc program, the more refined the final calc is. Taking shading into consideration will have major impact on heat gain. Try the HVAC Calc and see how it stacks up.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  4. #4
    Join Date
    Nov 2009
    Location
    Melbourne, FL
    Posts
    44
    Heaterman, the BTU number at the end was approx. 63,000. IS that what you're asking about?

    We will very likely go with a two-stage unit. Would that make a difference in what size we should have?

    Also-- so you are saying that some load calculators are better than others? That's what the one guy who didn't show me anything but said he did it said.


    Beenthere- Thanks for the link. I'll see if I can talk the hubby into paying the money for it!

    One more question-- The guy who showed me the numbers said he was doubtful that the other guy did a load calculation b/c he wasn't there for more than a half an hour before he talked $$ with me, and I didn't see him measuring anything (not saying he didn't, I just didn't see him do it). Do any of you agree with this?

    Thanks!!

  5. #5
    Join Date
    Apr 2008
    Location
    Northwest Arkansas via Chicago Area via Straight Up from There on Lake Superior
    Posts
    1,411
    Quote Originally Posted by ggcampbell View Post
    I recently received quotes from three different companies. Two of the companies said I needed a 5 ton system, while the other guy swears up and down I only need a 4 ton system.

    ...

    For what it's worth, my house is roughly 2450 sf. It faces east. I have three large sliders that make up the majority of the backside of my house, but I also have a large back porch with a roof, so I don't get much sun or heat in the back of the house. The guy who showed me his calculations said he wrote up my sliders as having window tinting. They don't have it, but he said he wrote it up like that to take into account the shading I receive from my porch roof.

    My current system is 18 years old and the info on it has worn off, so no one knows what I currently have.

    What would you be inclined to think?
    Do you know what size your current system is? All contractors should be able to tell you this. Did it adequately heat/cool your house before any problems? When I asked for proposals, I told all that they were NOT to exceed the current system size since the old 1-stage, on/off, AC/furnace adequately handled the house if not necessarily in the most comfortable way I would like from a more sophisticated multi-stage system. Did you limit any proposals to not-to-exceed current size and must be based on Manual J analysis that they must show you and provide to you (at least after the signing)?

    I'm just a HO, but a 4 or 5 ton system seems excessive for a house of 2450 sqft. Don't know where you are located or how tight your house is but I personally would hope for a significantly smaller system or insure it by investing in energy upgrades to my house. As an example, we have a fairly tight house with 2 stories, close to 50% larger than yours, no zones and a 4 ton system that very adequately handles the house. As an after thought a smaller system might have been adequate but many brands don't have 1/2 ton increments in the larger sizes.

    Do you have a utility company that could give you an "unbiased" blower door and infrared analysis to check out your house tightness? Our electric co-op did this for $100. This is very informative and points out many things that you can fix with just a caulk gun, gaskets/weatherstripping and maybe some more insulation in the roof; all things that are cheap to moderate cost compared to larger systems and higher operating costs.

  6. #6
    Join Date
    Nov 2009
    Location
    Melbourne, FL
    Posts
    44
    Jerryd -- thanks for your reply. Unfortunately, everyone who has checked out our unit has said they can't tell what size it is. I guess all the info has worn off?

    I'm in Florida, central/coastal. Not sure what kind of difference that makes.

    THanks for the idea on checking with the utility company. I'll see what they say.

  7. #7
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Quote Originally Posted by ggcampbell View Post
    the BTU number at the end was approx. 63,000. IS that what you're asking about?
    63,000 btu = 5.25 tons.

    Take care.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,743
    Can't do a load calc without measuring.
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  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,242

    Exclamation Give me a break

    Quote Originally Posted by beenthere View Post
    Can't do a load calc without measuring.
    Sure can with a trained eye ball for window and door area and paced off a 3' feet to step.

    Good enough for bid. Check prior to providing model numbers.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
    Join Date
    Jun 2003
    Location
    Jacksonville, FL.
    Posts
    4,313

    jerryd,

    with all due (or undue) respect you're the kind of HO I would hate to work for. You know the kind (because you are one): The one who refuses to let the contractor do the job right he's trained to do.

    I would go with the 5Ton, because the guy who did the heat load calc obviously knows what he's doing.
    WHY?

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,242

    Question 4 OR 5 TON

    Quote Originally Posted by ggcampbell View Post
    Heaterman, the BTU number at the end was approx. 63,000. IS that what you're asking about?

    We will very likely go with a two-stage unit. Would that make a difference in what size we should have?

    Also-- so you are saying that some load calculators are better than others?

    Thanks!!
    2,400 Sq. Ft with < 370 total Sq. ft of glass in Central FL. 18 years old

    W.A.G.N.I.R. : 51,000 BTU.

    63,000 seems >> H I G H <<
    Ceiling total ? ___ BTU/HR
    Glass ........ ____ BTU/HR
    Inside Temp 76' F ? [ N O T 72' F]
    Outside Temp 93' F

    If you want it < 76' F, 5-TON IS LIKELY NEEDED.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    Nov 2009
    Location
    Melbourne, FL
    Posts
    44
    Dan, my ceilings are 10 ft. I haven't measured glass but the master BR and LR face west and both have sliders (LR is oversized) with a transom window above. I also have oversized sliders on one wall of my FR, which faces south, and three large windows in the breakfast room that face S, SW, and W (my house is "L" shaped, hence the confusing configuration. Obviously I get light in b/c they are windows, but it is shaded by a roofed/screened patio that is connected to all aforementioned areas of my house. In most places, the roof extends out 10 or 12 feet behind the outer walls of my house. I don't have curtains or blinds up on any of these windows -- sunlight and heat are minimal. (My former house had a similar configuration in the breakfast room but did not have the enclosed porch extending beyond the house, and it got HOT. And that was even up north, not in Florida. So I think my porch must help out.

    As far as temp goes, all these sales guys are telling me that if I can get my humidity down, I can easily set my A/C to 76-79 and it will feel more like 72.

    Still trying to get the heat load calc from the one guy who claims he did it but never told me about until I asked a week later. I spoke with their office, and was told they use software instead of hand written calculations, and they told me they were more accurate and didn't take as long. This is the company that has me down for a 4 ton. Hmmmm.

    What is WAGNIR?

  13. #13
    Join Date
    Dec 2005
    Posts
    1,908
    Quote Originally Posted by ggcampbell View Post

    What would you be inclined to think?
    That's why when we were kid we used to say 2 out of 3.

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